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Influenza A H5N1

Most influenza viruses that infect wild or domestic birds cause limited illness in humans. However, viruses within the H5 and H7 subtypes have acquired genetic properties that have made them more virulent.

Highly pathogenic avian H5N1 influenza A viruses are endemic among bird and poultry populations in Asian countries and are considered the world's major influenza pandemic threat. The first association of avian influenza H5N1 with clinical respiratory disease occurred in Hong Kong in 1997 during a poultry outbreak of highly pathogenic H5N1 influenza in live-bird markets. This outbreak was associated with a high incidence of pneumonia and a 33% mortality rate. All virus genes were of avian origin, suggesting that H5N1 had jumped the species barrier without adaptation. Serologic surveillance revealed little evidence of human-to-human transmission, and no further cases were reported following mass culling of poultry.

Since its emergence in humans in 1997, influenza H5N1 has undergone antigenic drift. In addition to infecting poultry and humans, H5N1 virus appears to have extended its host range into felids. Fatal infection in domestic cats has been reported. There are no reports of cats transmitting the virus to humans. So far all human subtype H5 infections have resulted from viruses possessing the N1 subtype. Concern remains that this viral strain might mutate, or undergo reassortment with a human influenza virus, and acquire the ability to spread rapidly from human to human.

 On January 8, 2014, the Public Health Agency of Canada reported the first confirmed case of human infection with avian influenza A (H5N1) virus identified in North America. The patient exhibited symptoms while returning from travel to Beijing, China, on December 27, 2013.

 Avian influenza A H5N1 virus can infect and cause severe respiratory illness in humans. Most reports of H5N1 in humans have described severe illness including fulminant pneumonia leading to respiratory failure, acute respiratory distress syndrome, and death. Other reported H5N1 complications include encephalitis, septic shock, and multi-organ failure.

Clinicians should consider the possibility of avian influenza A (H5N1) virus infection in persons exhibiting symptoms of severe respiratory illness who have appropriate travel or exposure history. This includes persons who have traveled to endemic areas within 10 days of illness onset.

Patients who meet both the clinical and exposure criteria should be tested for avian influenza A (H5N1) virus infection by reverse-transcription polymerase chain reaction (RT-PCR) assay using H5-specific primers and probes. This testing is available in public health laboratories. Acceptable specimens include nasopharyngeal, nasal, or throat swabs using a Dacron/flocked swab and any commercially available viral transport media. Tracheal aspirate and bronchoalveolar lavage (BAL) specimens could be submitted as well Specimens must be stored at 2-8 °C and shipped on frozen refrigerant packs within three days OR stored at -70°C and sent on dry ice if held longer than 3 days.

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